Colorless combiners and multiplexers are used to combine wavelengths. For example, in a dynamic wavelength router (DWR) variable optical attenuator mux module (VMUX) a colorless combiner is used to colorlessly combine local add channels. A colorless combiner is also used in a booster-combiner amplifier to colorlessly combine local add channels with express channels from other node degrees.
However, a risk in colorless multiplexing is the potential for the addition of incorrect channel wavelengths that are already in use by working, traffic-carrying, channels, thus causing wavelength collisions and data errors. Such incorrect wavelengths may occur due to errors in tunable-wavelength settings, or by erroneous choice of fixed-wavelength data cards. If the incorrect wavelength is permitted to ramp up in power, then interference will occur between local add channels, or between add and express channels, even when using colored VMUX modules (if they are subtending to a wavelength selective service (WSS)). See, for example, in FIG. 1, wherein a reconfigurable optical add-drop multiplexer (ROADM) node is shown, illustrating modules that are colorless combiners (elements 114, 116), and further illustrating potentially interfering paths (i.e. the dashed lines).
Modulation of data channels is well known in the art. For example, modulation of data channels is used to identify working channels, to quantify parameters in working channels, and to carry ancillary information. U.S. Pat. No. 5,390,185, issued to Hooijmans et al. on Feb. 14, 1995, discloses a transmission system that includes a transmitter for transmitting to a communication channel a send signal, which is a combination of a main signal and an auxiliary signal. U.S. Pat. No. 5,798,857, issued to Tamura et al. on Aug. 25, 1998, discloses spontaneous emission occurring in association with direct optical amplification using an erbium-doped optical fiber that is modulated with an auxiliary signal, such as a supervisory signal, for transmission of the auxiliary signal. U.S. Pat. No. 5,995,256, issued to Fee on Nov. 30, 1999, discloses a method and apparatus that are provided for high signal-to-noise ratio (SNR) optical subcarrier management and reception in a communication network. U.S. Pat. No. 6,108,113, issued to Fee on Aug. 22, 2000, discloses a method and system for transporting ancillary network data wherein a sub-carrier modulation signal containing ancillary network data is superimposed on a high bit-rate data signal prior to transport over an optical link. Additionally, U.S. Pat. No. 6,574,016, issued to Harley et al. on Jun. 3, 2003, discloses a method and apparatus for ancillary data in a wavelength division multiplexed (WDM) system wherein a low bit rate channel is provided over an amplitude modulated sub-carrier that is used to amplitude intensity modulate an optical data signal that is output from a transmitter in the network. Finally, U.S. Pat. No. 7,054,556, issued to Wan et al. on May 30, 2006, discloses the use of fast Fourier Transform (FFT) detection of channel modulation for working traffic channel identification.
Also known in the art is the method of using colored multiplexors (filters) to prevent incorrect wavelengths from being added. However, if colored multiplexors (filters) are introduced, then the system is no longer a colorless multiplex system. Additionally, another method known in the art is to try and detect new channels by a direct power reading before bringing the later channel up to full power. Background light often makes a direct power measurement insensitive, so that the power needed to detect a new channel is higher than the threshold for causing data errors in existing traffic.
The patents disclosed above demonstrate how information may be carried continuously via various modulations techniques, or identify wavelengths in channels at working power. None of these systems or methods, however, are used to verify the suitability of an optical wavelength for addition to an optical system before it is made operational and brought to potentially damaging power, because they lack the ability to cross-correlate modulation information with the physical wavelength selectivity of the Optical Channel Monitor (OCM).
Thus, what is needed is a system and method for detecting new optical channels at powers below that which would cause data errors in existing traffic. Furthermore, what is needed is a system and method for detecting new optical channels at these power levels while on top of background light.